It's the continuing push and pull of Loki and Thor's relationship. That's really the film.

IDA with that. Yes, it's in there and is a factor towards the last part but until

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the break in of Malekith, Thor didn't even think of Loki. He was going through the motions of securing the realm and moping about Jane. He and Loki hadn't even seen each other since he delivered him to Odin. Honestly, I don't know that there is a central "theme" to this movie which could be one of the problems. Once they left Asgard, they got their talk but to me it fell flat. Loki is jealous of Thor blah blah blah. We know that already. Even Loki's sacrifice wasn't deep. I knew before the movie got going he would do anything to keep Thor off the throne if given the opportunity.